High voltage beam tube having spark inhibiting contacting surfaces



NOV 28, 1957 R. L. GEIGER ETAL 3,355,614

HIGH VOLTAGE BEAM TUBE HAVNG SPARK INHIBITING CONTACTING SURFACES Filed OCT.. 4, 1965 INVENTORS wenn@ Lem-eea cS- MARsHnLL RcLELAND 'WMV ATTORNEYS United States Patent O 3,355,614 HIGH VOLTAGE BEAM TUBE HAViNG SPARK INHIBITING CONTACTNG SURFACES Richard L. Geiger, Summit, NJ., and Marshall R.

Cleland, Huntington Station, N.Y., assignors to Radiation Dynamics, Inc., Westbury, N.Y., a corporation of New York Filed st. 4, 1963, Ser. No. 313,868 7 Claims. (Cl. 313-63) The present invention relates to high-vacuum, highvoltage, electron and ion beam acceleration tubes and, more particularly, to a construction of a high-vacuum, high-voltage electron and ion beam acceleration tube which minimizes sparking within the tube and which permits the generation of high current ion and electron beams.

The high-vacuum, high-voltage acceleration tubes of the type with which the present invention is concerned have alternately arranged and axially aligned metallic discs and hollow glass cylinders bonded one to the other.

Actually the present invention is not limited to metallic discs or glass cylinders. The discs may be made of various conductive materials and the cylinders may be made of other types of insulators of the proper conductivity and where the terms metallic discs and glass cylinders are employed they are to be considered in the light of this statement.

The metallic discs extend both internally and externally of the glass cylinders and are centrally apertured so as to provide a path for an electron or ion beam. A hot cathode or ion source structure is arranged at one end of the column formed by the alternate sections of hollow cylinders and apertured metallic discs while a target structure or electron-permeable window is provided at the other end of the column. The metallic discs are electrically connected to a voltage divider such that an accelerating field is established along the column in a direction away from the source structure.

It is conventional in such tubes to provide conductive collars disposed in the apertures of and connected to each of the conductive discs so as to shield the electron or ion beam from nonuniform electrostatic charges which may accumulate on the inner glass surfaces of the glass cylinders. The accumulation of charge on the inner surface of the cylinders does not become a serious problem when the tube is employed with low beam currents. However where large beam currents are to be accelerated bythe tube; that is beam currents equal to or greater than 1 milliampere, the accumulation of charge on the glass due to beam scattering becomes great enough to produce uncontrollable deliection of the beam rendering focusing and aiming of the beam difficult if not impossible. The accumulation of charge may be reduced to an acceptable value by employing glass or other insulating material having a resistivity which is low enough to permit a flow of current sulicient to substantially drain olf the scattered electrons as they strike the cylinder. In other words a dynamic balance of flow to and from the cylinder is established such that the charge accumulated is maintained below troublesome values. The resistivity of the glass, etc.,

fcannot be too low or the high voltage supply would be unduly loaded and therefore an intermediate resistivity glass is employed; that is, a glass having a volume resistivity of between 1011 to 1014 ohm cm. It is found that in the systems involved the current through the cylinder runs about l to 10 microamperes, a flow which has been lfound to be large enough to maintain the charge accumulation at acceptable levels in systems employing beam currents of from 1 to 10 milliamperes.

In the fabrication of such tubes, it is conventional to bond the edges of the glass cylinders and the mating sur- ICC faces of the metal discs, one t-o the other, by means of a synthetic resin to provide a vacuum-tight seal so that a high vacuum may be maintained within the column. It is essential that a minmum of this resin be located internally of the tube where it may be bombarded by stray particles, such as electrons and ions. Bombardment of the glue or resin by electrons and/or ions causes this latter material to become decomposed. Decomposition of the material liberates gas in the vacuum chamber and leaves a carbonaceous residue which may become deposited in specks over the interior surfaces of the tube, materially detracting from the operation thereof. In accordance with an invention described in co-pending patent application Ser. No. 124,928, filed July 18, 1961, by Marshall R Cleland now Patent No. 3,178,601 and assigned to the assignee of the present invention, the bonding material is applied by causing the resin to flow between the metallic disc and its associated hollow glass cylinders by means of capillary action. The metallic discs or the adjacent surfaces of the glass cylinders are undercut so as to terminate the capillary flow radially outward of the inner surface of the hollow glass cylinders. Thus, it is assured that substantially none of the resin is carried into a region where it is subject to ion or electron bombardment.

The tube st-ructure defined above proved highly satisfactory in operation but, as it was desired to raise the voltage across each pair of metal discs so as to increase the total Voltage which could be applied across a tube of given length, internal sparking and discharges occurred which effectively destroyed the ability of the tube to operate at these higher voltages. In a specific tube tested, the axial length of a hollow glass cylinder formed of soda-lime glass was one and three-eights inches and the discharges began to occur within the tube when voltages of over 23 kilovolts were applied between the metal discs disposed at each end of the hollow cylinder. It Was noted that most of the sparking occurred between the sharp edge of the glass cylinder provided at the intersection of its end surface and its inner circumferential surface and the adjacent metallic surface of the metallic disc. These two parts are spaced apart in this region as a result of the undercut in the metallic disc.

It was hoped that the sparking could be eliminated by removing the sharp edge of the glass cylinder by beveling the edge. This was done; the tube was reassembled; and voltage was applied. The improvement achieved was very slight, discharge still occurring between the metal cylinder and the hollow glass cylinder and also some discharge appearing between the bevels at the two ends of the hollow glass cylinder along the inner circumferential surface thereof.

The internal arcing of tubes of this character has been a constant dilhculty in the design and construction of linear accelerator tubes and it was decided to undertake a study to determine the basic nature of the discharges in hopes of greatly increasing the permissible operatingvoltage range rather than employ hit-or-miss techniques which might gradually raise the discharge limit may be 5 or 10 percent.

After some thought was given to the subject, it occurred to us that perhaps the resins or glues employed to provide the vacuum seal were responsible for some of the diiculty and a tube section was assembled with no material present between the glass cylinder and the metal disc; the vacuum seal being provided by a putty or wax formed around the exterior of each of the joints between the various elements of the tube. The tube was evacuated and voltage applied. Arcing and sparking occurred at relatively low voltages and considerably below that of the structure of my prior invention.

After continued research and experimentation, a theory was evolved that perhaps current flow through the hollow glass cylinders, although very small, was responsible for accumulations of charge along the end surface of the cylinder at the interface between the glass cylinder and the metal disc. As indicated above, the glass employed in tubes of the type with which the present invention is concerned is a relatively low resistivity glass; such as l012 ohm-cm. at 25 C. The low resistivity glass is employed in order to reduce to an acceptable quantity the static voltage build-up on the inner surface of the glass cylinders due to electron or ion bombardment of the interior surface of the cylinder. The effects of current iiow through the glass was then considered in light of the nature of the interface between the glass and metal. The mating glass and metal surfaces, although quite smooth to the eye, are, on a microscopic basis, quite rough and points, rather than surfaces, of contact are provided. It occurred to us that, if the resistance along the end surface of the glass between a point of contact with the metal disc and a point at which charge was accumulating was high enough, a dynamic balance could not be obtained and the charge would accumulate at a faster rate than it could iow olf, resulting in breakdown of the vacuum gap between the point of charge accumulation and the metal disc.

Returning to the case of the undercut metal disc and applying the above theory, the impedance of the glass along its bottom surface was evidently large enough to permit a suriicient accumulation of charge at the outer edge of the glass cylinder to produce breakdown of the vacuum gap between the metal disc and the edge of the glass cylinder. It was postulated then that the operation of this type of apparatus could be improved by increasing the conductivity of the glass along its end surfaces to prevent the build-up of local charges of sumcient magnitude to produce arcing.

The experiment indicated above, wherein the glass and metal were held in intimate contact under pressure and a wax was applied around the outside of the joint to support the vacuum, was repeated except this time, the end surfaces of the hollow glass cylinder were coated with Aquadag. Voltage was applied between the metal discs abutting the opposite ends of the hollow glass cylinder and voltage discharge in the apparatus did not occur until the voltage thereacross reached approximately 8O kilovolts. This is an increase of three and one-half times the voltage which could be maintained across such an apparatus as previously constructed; that is, without employing the teaching of the present invention.

It then occurred to us that basically what was needed was to prevent charge accumulation by any mechanism and that this could also -be achieved by insuring that there were no electrical discontinuities between the glass and metal so that all current flowing through the glass cylinders would flow directly to the metal disc. Since the mating glass and metal surfaces cannot be ground to accomplish such a result, it was decided to employ an intermediate resistivity resinous sealer between the glass and metal; that is, a resin having a resistivity equal to `or less than the resistivity of the glass. Further, an attempt must be made to eliminate all voids in the system. This approach was tried and, although the performance of this structure was not as good as when a conductive coating was applied to the ends of the glass, a voltage of 70 kilovolts was established across the structure before arc discharges occurred. The lesser performance of this device is probably due to the inability to eliminate all voids at the glass-to-metal interface.

Each of the two system described above has its advantages and disadvantages. The system employing only the conductive resin does not require the additional step of coating the ends of the glass cylinders. However, in manufacture, the care required in practicing the invention in .order to minimize voids in the system may prove tedious. Also, in this system, the method of the aforesaid .Cleland application may prove difficult to employ.

The system employing the conductive coatings on the ends of the glass cylinder requires an extra manufacturing step but, in all `other respects, lends itself to rapid fabrication techniques.

lt has been stated that the ends of the glass cylinders are coated with Aquadag lbut other conventional techniques may be employed for rendering the ends of the hollow glass cylinders conductive. T he ends may be metallized by evaporation techniques, by sintered powder metal, or by hydrogen ring of metal oxides. The -bontling agents which are of the same or lower resistivity than the glass may be conventional glass-to-rnetal seals, powdered glass bands which are tired and fused, or organic resins may be employed. The resins set forth in the aforesaid co-pending application .of the epoxy type are particularly suitable in the present tubes so long as those resins are selected that have the necessary electrical characteristics.

All of the techniques employed in the aforesaid copending application may be employed in manufacturing the tubes of the first embodiment of the present invention. Either the metal discs or glass cylinders may be circumferentially grooved to provide a capillary termina tion point and after assembly, the assembled discs and cylinders are put into a compression unit, the liquid epoxy resin is applied to the exterior surface and permitted to cover a desired portion of the mating surfaces and allowed to cure at room temperature. The glass and metal stack lmay then be assembled with the cathode and end structures and clamped together with cinch rods. Other suitable assembly techniques may be employed. It should be noted however that the interior undercut mentioned above may be employed at lesser voltage. For instance, it is possible to apply, by painting, an annulus of the cement only over the outer half of the annulus provided by the glass cylinder. In this way, since the cylinder walls are of the order of one-half an inch thick, it is an easy matter to prevent the resin from running by accident into the interior of the apparatus.

With regard to the second embodiment of the invention, a bonding resin may be applied to the ends of the glass cylinder, making certain that little or substantially none of the material becomes deposited on the inner surface thereof. Also, capillary action may be employed. In this system, no undercuts may be employed so that an internal fillet of resin may be formed. This may be subsequently washed away with a solvent, mak-ing certain `that none of the resin in the interface is removed.

It is apparent from the above that, in order to provide a linear acceleration tube of the type with which the present invention is concerned, a good electrical Contact must be maintained between the ends of the hollow glass cylinders and the metal electrodes. Further, the glass employed must be an `intermediate-resistivity glass as defined hereinabove.

It is an object of the present invention to provide a high-voltage, high-vacuum electron and ion bea-m acceleration tube which is constructed of alternately arranged hollow glass cylinders and apertured metal discs, which apparatus can withstand unusually high voltages across adjacent metal discs.

Another object of the present kinvention is to provide a high-voltage, high-vacuum electron and ion beam acceleration tube employing alternate sections of intermediate resistivity, hollow glass cylinders and apertured metal discs wherein electrical contact is maintained `throughout the interface between the glass and metal.

Yet another object of the present invention is to provide a high-voltage, high-vacuum electron and ion beam acceleration tube employing alternate sections of intermediate-resistivity, hollow glass cylinders and apertured metal discs wherein electrical contact is maintained throughout the interface between the glass and metal by means of a bonding agent having a resistivity equal to or less than the resistivity of the glass.

It is another object of the present invention to provide a high-voltage, high-vacuum electron and ion beam acceleration tube employing alternate se-ctions of intermediate resistivity, hollow glass cylinders and apertured metallic discs wherein the end surfaces of the hollow glass cylinders are coated with a highly conductive material and thereafter cemented to the adjacent metal discs so as to provide an apparatus capable of withstanding an unusually high voltage across the glass cylinder.

It is another object of the present invention to provide a high-voltage, high-vacuum ele-cron and ion beam acceleration tube having alternately arranged hollow glass cylinders and apertured metal discs which lends itself to simple methods of fabrication such as those disclosed in the aforesaid co-pending patent application, Ser. No. 124,928 now Patent No. 3,178,601.

Yet another object of the present invention is to provide a high-voltage, high-vacuum electron and ion beam acceleration tube employing alternately arranged hollow glass cylinders of intermediate resistivity glass and apertured nie-tal discs wherein each end surface of each glass cylinder is equi-potential.

The above an-d still further objects, features and advantages of the present invention will be come apparent upon consideration of the following detailed description of several specific embodiments thereof, especially when taken in conjunction with the accompanying drawings, wherein:

FIGURE l is a longitudinal, sectional view of the beam tube of the present invention with the portion intermediate the two ends thereof broken away;

FIGURE 2 is an enlarged sectional view of the region of the tube adjacent two of the interfaces between the glass cylinders and a metal disc;

FIGURE 3 is an enlarged sectional View of a modified interface according to the present invention;

FIGURE 4 is an enlarged sectional View of an interface between a glass cylinder and a metal disc illustrating an annular groove formed in the end face of the glass cylinder to terminate capillary action; FIGURE 5 is an enlarged sectional View of an interface between a glass cylinder and a metal disc illustrating a pair of annular notches formed in opposite faces of the disc to terminate capillary action; and

FIGURE 6 is an enlarged sectional view of an interface between a glass cylinder and a conductive disc illustrating a further embodiment of the present invention.

Referring now specifically to FIGURE 1 of the accompanying drawings, the beam tube comprises a plurality of intermediate resistivity, hollow glass cylinders 1, alternately arranged and axially aligned with conductive discs 2 having centrally located apertures 3. The number and size of the hollow glass cylinders 1 and discs 2 is the function of the beam current yand the voltage of the apparatus. For purposesof example only, it is assumed that the glass cylinders are fabricated from soda-lime glass and have an axial length of approximately 1%". The metallic discs 2 may be fabricated from titanium, stainless steel or other conductive material and, in the preferred form of the invention, have approximately the same temperature coefficient of expansion as the glass cylinders 1.

The assembly is provided with a generally square flange 5 adjacent the target region in contact with an end disc 4. A generally circular end flange 6 is provided adjacent the cathode end of the'tube and in contact with a metallic end disc 7. A cathode assembly 8 is positioned below, as viewed in FIGURE 1, the square end tiange 6 and is secured thereto, for instance, by four polyester-impregnated ber glass cinch rods 9 which extend to the end plate 5 to secure the entire assembly together. Metal collars 10 may be located in the apertures 3 as indicated in the aforesaid co-pending application so Ias to reduce the number of scattered ions and electrons which strike the interior walls of the hollow glass cylinders and to shield the beam from stray electric fields.

In accordance with one embodiment of the present invention, the ends of each of the hollow glass cylinders are coated with la conductive film, such as 11, which is maintained in intimate contact with the adjacent surfaces of the metal discs 2, 4 and 7. Referring specifically to FIGURE 2 of the accompanying drawings, the details of a single joint are illustrated in greatly enlarged section so as to more clearly indicate the nature of the invention. As previously indicated, the ends `of the hollow glass cylinders are provided with a conductive film 11 which may be Aquadag or an evaporated, sintered or fired conductive material. A suitable bonding agent 12 is provided for sealing the region between the end of each glass annulus 1 and the adjacent metallic disc 2. The cemented joint helps to impart mechanical strength to the apparatus and also provides the vacuum seal required when the interior oi the tube is evacuated during use.

Various types of joint structures may be readily ernployed with the apparatus of the invention and referring to FIGURE 3, a further type of arrangement is illustrated. The end of a hollow glass cylinder 13 is provided with a conductive coating 14 and the bonding agent is applied initially to a region 16 on an adjacent metal disc 17 or on the end of the glass cylinder. The region 16 is an annular region extending between the outer circumference of the glass cylinder 13 to la distance approximately half way between its inner and outer circumferences. Thus, a vacuum seal is maintained over the outer half of the annulus land an electrical contact between the coating 14 and the metal disc 17 is maintained over the inner half of the lannulus formed by the wall of the hollow cylinder 13. In such an arrangement the placement of the cement material 16 is not critical since small deviations in its placement do not bring it int-o the interior of the tube where it can be .bombarded by ions or electrons.

Referring specically to FIGURE 4 of the accompanying drawings, there is illustrated an arrangement which permits the techniques of fabrication employed in the aforesaid co-pending patent application to be employed herein. A hollow glass cylinder 18 has annular grooves 19 formed in its end walls. The entire end surface of the hollow glass cylinders are 4again metallized; that is, provided with a conductive coating 21 which also extends into and coats the surfaces of the grooves 19. The end surfaces of the cylinder which contact a metallic disc 22 are very smooth, the mating surfaces of the disc and cyl- -inder supporting capillary action therebetween.

The groove 19 is employed to terminate the capillary action by increasing the distance between the disc 22 and its adjacent glass surface to such an extent that capillary action terminates some place in the groove. A room-curing liquid epoxy resin is applied to the outer edge of the interface between the disc [and the cylinder and, due to the capillary action, flows between the two members until it reaches the groove. Thus, an effect similar to that of FIG- URE 3 is :accomplished except that it is not required to paint the cement on the metal disc. It is merely required to .apply the cement to the exterior of the interface between the two members and permit it to flow therebetween. The conductive coating 21 must extend across the entire end face of the glass cylinders 18 so las to prevent local areas of charge, particularly in the region of the groove 19.

Referring now specifically to FIGURE 5 of the accompanying drawings, an arrangement is illustrated which is very similar to that of FIGURE 4 except that the groove for terminating capillary action is provided in the metallic disc rather than in the glass cylinder. Specifically, a hollow glass cylinder 23 h-as metallized end surfaces to provide conductive coatings, such .as the coating 24. The end surfaces of the hollow glass cylinder are in intimate contact with metallic discs such :as the disc 26, the opposite faces of which are grooved as at 27. Again, due

to capillary action, 'an epoxy resin is drawn between the two members to provide bonding in an area designated by the reference numeral 23 which area is terminated by the groove 27 which prevents further capillary attraction of the material into the interface.

The metal discs may be undercut to terminate capillary action as taught in the aforesaid application. This method is in the alternative to those of FIGURES 4 4and 5 of the accompanying drawings.

Referring now specifically to FIGURE 6 of the accompanying drawings, there is illustrated still 'another embodiment of the present invention. The apparatus comprises an intermediate resistivity hollow, glass cylinder 29 and a conductive or metallic disc 31 having the same general construction as illustrated in FIGURES 1 through 4. A layer of bonding material 32 is disposed between the glass cylinder 29 and the conductive disc 31 so as to provide a vacuum seal between these two members. The bonding agent 32 has a resistivity equal to or less than resistivity of the glass from which the cylinder 29 is fabricated.

As previously indicated, current ow through the glass cylinder 29, due to the maintenance of a high voltage thereacross, will accumulate at the interface between the cylinder and disc unless a substantially continuous electrical contact is established between these two members. Therefore, the bonding material 32 must support the same current flow therethrough as the glass and must also be substantially free of voids so that substantially no discontinuities in flow are present.

Various methods may be employed for applying the bonding material to the interface. Thus, the end of the glass cylinder -may -be coated and then applied to the metal disc. Further, the metal disc and glass cylinder may be clamped together and sealed by bonding material permitted to flow through the interface by means of capillary action. In either of these instances, a fillet may be formed at the interface. If such is the case, this fillet may be removed by washing with a solvent; care being taken not to remove any of the material in the region between the glass cylinder and the metallic disc. It should be noted that where the ends of the glass cylinders are metallized, the epoxy may be washed away since a charge will not build up on the glass in such a region. Washing away of the resin becomes a problem only where the system relies on the resin to prevent the -build-up of charge which is not the case where a conductive coating is employed.

An important aspect of the present invention is the maintanence of equi-potential end surfaces of the glass cylinders either by metallizing the ends of the hollow glass cylinders or by employing an intermediate resistivity bonding agent. The metallized or conductive ends surfaces of the cylinders permit the current flow through the cylinders without causing local areas of high static charge to be created; that is, the ends of the cylinders present equi-potential surfaces to the metal discs. The intermediate resistivity bonding agent produces substantially the same result by permitting current llow to the disc to be maintained across the entire interface. If local areas of high potential are permitted they would produce arc discharges to the closest metal surface which would eventually cause contamination of the tube and prevent the full voltage build-up across each of the interfaces between the glass cylinders and the metal discs.

Although a full tube construction is not illustrated, it is contemplated that conventional structures may be employed. Also, the construction illustrated in my aforesaid co-pending application may be utilized with the only changes relating to the interface regions as specifically set forth herein.

While I have descri-bed and illustrated several specific embodiments of my invention, it will be clear that variations of the details of construction which are specically illustrated and described may be resorted to without departing from the true spirit and scope of the invention as delined in the appended claims.

What we claim is:

1. A high-voltage tube structure for accelerating ion and electron beams comprising a plurality of centrally apertured, thin, metallic discs, a plurality of hollow glass cylinders having annular end surfaces, said discs and cylinders being arranged in interleaved and abutting relationship along a common axis to provide a columnar structure, a conductive coating disposed on the end surfaces of each of said cylinders, and a bonding agent for sealing said metal discs to said conductive coatings.

2. A high-voltage tube structure for accelerating ion and electron beams comprising a plurality of centrally apertured, thin, conductive discs, a plurality of intermediateresistivity hollow insulating cylinders having annular end surfaces, said discs and cylinders being arranged in interleaved and abutting relationship along a common axis to provide a columnar structure, a conductive coating disposed on the end surfaces of each of said cylinders, and a bonding agent applied between each of said conductive coatings and the adjacent conductive discs.

3. A high-voltage tube structure for accelerating ion and electron beams comprising a plurality of centrally apertured, thin, metallic discs, a plurality of hollow glass cylinders having annular end surfaces, a conductive coating disposed on the end surfaces of each of said cylinders, said discs and cylinders being arranged in interleaved relationship along a common axis with said conductive coatings in direct physical and electrical contact with said discs, and a bonding agent filling the voids between the points of contact of said discs and said conductive coatings.

4. The combination according to claim 3 wherein said bonding agent is a room-temperature curing epoxy resin.

5. The combination according to claim 4 wherein each of said metallic discs has an annular groove formed at a radius lying between the inner and outer radii of the hollow cylinders.

6. The combination according to claim 4 wherein said end surfaces of said hollow cylinders have an annular groove intermediate the inner and outer circumferences thereof.

7. The combinationv according to claim 3 wherein said bonding agent extends from at least the outer circumference of said cylinders to locations having a greater radius than the inner circumference of said cylinders.

References Cited UNITED STATES PATENTS 2,460,201 1/1949 Trump et al.v 313-313 2,517,260 8/1950 Van de Graaf et al. 313-82 2,714,679 8/1955 Van de Graaf et al. 313-313 2,828,235 3/1958 Holland et al. 161-186 3,141,058 7/1964 Titus 313-313 3,178,601 4/1965 Cleland 313-63 JAMES W. LAWRENCE, Primary Examiner.

V. LAFRANCHI, Assistant Examiner. 

1. A HIGH-VOLTAGE TUBE STRUCTURE FOR ACCELERATING ION AND ELECTRON BEAMS COMPRISING A PLURALITY OF CENTRALLY APERTURED, THIN, METALLIC DISCS, A PLURALITY OF HOLLOW GLASS CYLINDERS HAVING ANNULAR END SURFACES, SAID DISCS AND CYLINDERS BEING ARRANGED IN INTERLEAVED AND ABUTTING RELATIONSHIP ALONG A COMMON AXIS TO PROVIDE A COLUMNAR STRUCTURE, A CONDUCTIVE COATING DISPOSED ON THE END SURFACES OF EACH OF SAID CYLINDERS, AND A BONDING AGENT FOR SEALING SAID METAL DISCS TO SAID CONDUCTIVE COATINGS. 